encompasses the statistics of temperature
particle count and numerous other meteorological
elements in a given region over
long periods of time. Climate can be contrasted to weather
, which is the present condition of these
same elements over periods up to two weeks.
The climate of a location is affected by its latitude, terrain,
altitude, ice or snow cover, as well as nearby water bodies and
their currents. Climates can be classified
according to the average
and typical ranges of different variables, most commonly
temperature and rainfall. The most commonly used classification
scheme is the one originally developed by Wladimir Köppen
. The Thornthwaite
system, in use since 1948, incorporates evapotranspiration
in addition to
temperature and precipitation information and is used in studying
animal species diversity and potential impacts of climate changes
. The Bergeron and Spatial Synoptic
focus on the origin of air masses
defining the climate for certain areas.
is the study and
description of ancient climates. Since direct observations of
climate are not available before the 19th century, paleoclimates
are inferred from proxy variables
that include non-biotic
evidence such as sediments found in lake beds and ice cores, and
biotic evidence such as tree rings and coral. Climate models
are mathematical models of
past, present and future climates.
Climate(from Ancient Greek
, meaning inclination
) is commonly defined
as the weather averaged over a long period of time. The standard
averaging period is 30 years, but other periods may be used
depending on the purpose. Climate also includes statistics other
than the average, such as the magnitudes of day-to-day or
year-to-year variations. The Intergovernmental
Panel on Climate Change
(IPCC) glossary definition is:
The difference between climate and weather is usefully summarized
by the popular phrase "Climate is what you expect, weather is what
you get." Over historical
time spans there
are a number of nearly constant variables that determine climate,
including latitude, altitude, proportion of land to water, and
proximity to oceans and mountains. These change only over periods
of millions of years due to processes such as plate tectonics
. Other climate
determinants are more dynamic: for example, the thermohaline circulation of the
ocean leads to a 5 °C (9 °F) warming of the northern
ocean compared to other ocean basins.
between land and water on a more regional scale. The density and
type of vegetation coverage affects solar heat absorption, water
retention, and rainfall on a regional level. Alterations in the
quantity of atmospheric greenhouse
determines the amount of solar energy retained by the
planet, leading to global warming
. The variables which
determine climate are numerous and the interactions complex, but
there is general agreement that the broad outlines are understood,
at least insofar as the determinants of historical climate change
There are several ways to classify climates
regimes. Originally, climes
were defined in
to describe the
weather depending upon a location's latitude. Modern climate
classification methods can be broadly divided into genetic
methods, which focus on the causes of climate, and empiric
methods, which focus on the effects of climate. Examples of genetic
classification include methods based on the relative frequency of
different air mass
types or locations
within synoptic weather disturbances. Examples of empiric
classifications include climate zones defined by plant hardiness,
evapotranspiration, or more generally the Köppen climate
which was originally designed to identify the
climates associated with certain biomes. A common shortcoming of
these classification schemes is that they produce distinct
boundaries between the zones they define, rather than the gradual
transition of climate properties more common in nature.
Bergeron and Spatial Synoptic
Source regions of global air
The most generic classification is that involving the concept of
air masses. The Bergeron
is the most widely accepted form of air mass
classification. Air mass classification involves three letters. The
first letter describes its moisture properties, with c used for
continental air masses (dry) and m for maritime air masses (moist).
The second letter describes the thermal characteristic of its
source region: T for tropical, P for polar, A for Arctic or
Antarctic, M for monsoon, E for equatorial, and S for superior air
(dry air formed by significant downward motion in the atmosphere).
The third letter is used to designate the stability of the
atmosphere. If the air mass is colder than the ground below it, it
is labeled k. If the air mass is warmer than the ground below it,
it is labeled w. While air mass identification was originally used
in weather forecasting
the 1950s, climatologists began to establish synoptic climatologies
based on this idea in 1973.
Based upon the Bergeron classification scheme is the Spatial Synoptic
(SSC). There are six categories within
the SSC scheme: Dry Polar (similar to continental polar), Dry
Moderate (similar to maritime superior), Dry Tropical (similar to
continental tropical), Moist Polar (similar to maritime polar),
Moist Moderate (a hybrid between maritime polar and maritime
tropical), and Moist Tropical (similar to maritime tropical,
maritime monsoon, or maritime equatorial).
Monthly average surface temperatures
This is an example of how climate varies with location and
The Köppen classification depends on average monthly values of
temperature and precipitation. The most commonly used form of the
Köppen classification has five primary types labeled A through E.
Specifically, the primary types are A, tropical; B, dry; C, mild
mid-latitude; D, cold mid-latitude; and E, polar. The five primary
classifications can be further divided into secondary
classifications such as rain forest
, tropical savanna
, humid subtropical
, humid continental
, oceanic climate
, Mediterranean climate
, polar ice cap
, and desert
are characterized by high rainfall
, with definitions setting minimum normal
annual rainfall between and . Mean monthly temperatures exceed
during all months of the year.
is a seasonal prevailing wind which
lasts for several months, ushering in a region's rainy season.
Regions within North America
, South America
. Sub-Saharan Africa
are monsoon regimes.
A tropical savanna
is a grassland biome
climate regions of subtropical
, with average temperatures
remain at or above year round and rainfall between and a year.
widespread on Africa, and are also found in
India, the northern parts of South America, Malaysia, and
The humid subtropical
climate zone where winter
rainfall (and sometimes snowfall
associated with large storms
that the westerlies
steer from west to east. Most summer
rainfall occurs during thunderstorms
and from occasional tropical
. Humid subtropical climates lie on the east side
continents, roughly between latitudes
and 40° degrees away from the equator.
A humid continental
climate is marked by variable
weather patterns and a large seasonal temperature variance. Places
with a hottest monthly temperature above and a coldest month
temperature below and which do not meet the criteria for an
, are classified as
An oceanic climate
is typically found along the
west coasts at the middle latitudes of all the world's continents,
and in southeastern Australia
, and is
accompanied by plentiful precipitation year round.
Mediterranean climate regime resembles the climate
of the lands in the Mediterranean
Basin, parts of western North
America, parts of Western and South Australia, in southwestern South
Africa and in parts of central Chile.
climate is characterized by hot, dry summers and cool, wet
is a dry grassland
with an annual temperature range in the
summer of up to and during the winter down to .
A subarctic climate
has little precipitation, and
monthly temperatures which are above for one to three months of the
year, with continuous permafrost
the very cold winters. Winters within subarctic climates include up
to six months of temperatures averaging below .
Tundra occurs in the far
Hemisphere, north of the taiga belt,
including vast areas of northern Russia and Canada
A polar ice cap
, or polar ice sheet, is a
region of a planet
is covered in ice
. Ice caps form because
regions receive less energy
in the form of solar radiation
regions, resulting in lower surface
is a landscape
form or region that receives very little precipitation
. Deserts usually
have a large diurnal
and seasonal temperature range, with high daytime temperatures (in
summer up to 45 °C or 113 °F), and low night-time temperatures (in
winter down to 0 °C; 32 °F) due to extremely low humidity
. Many deserts are formed by rain shadows
, as mountains block the path of
moisture and precipitation to the desert.
Precipitation by month
Devised by the American climatologist and geographer C. W.
, this climate
classification method monitors the soil water budget using the
concept of evapotranspiration. It monitors the portion of total
precipitation used to nourish vegetation over a certain area. It
uses indices such as a humidity index and an aridity index to
determine an area's moisture regime based upon its average
temperature, average rainfall, and average vegetation type. The
lower the value of the index in any given area, the drier the area
The moisture classification includes climatic classes with
descriptors such as hyperhumid, humid, subhumid, subarid, semi-arid
(values of -20 to -40), and arid (values below -40). Humid regions
experience more precipitation than evaporation each year, while
arid regions experience greater evaporation than precipitation on
an annual basis. A total of 33 percent of the Earth's landmass
is considered either arid of semi-arid, including southwest North
America, southwest South America, most of northern and a small part
of southern Africa, southwest and portions of eastern Asia, as well
as much of Australia. Studies suggest that precipitation
effectiveness (PE) within the Thornthwaite moisture index is
overestimated in the summer and underestimated in the winter. This
index can be effectively used to determine the number of herbivore
numbers within a given area. The index is also used in studies of
Thermal classifications within the Thornthwaite scheme include
microthermal, mesothermal, and megathermal regimes. A microthermal
climate is one of low annual mean temperatures, generally between
and which experiences short summers and has a potential evaporation
between and . A mesothermal climate lacks persistent heat or
persistent cold, with potential evaporation between and . A
megathermal climate is one with persistent high temperatures and
abundant rainfall, with potential evaporation in excess of .
Instrumental temperature record of the
last 150 years
Details of the modern climate record are known through the taking
of measurements from such weather instruments as thermometers
, and anemometers
during the past few centuries. The
instruments used to study weather conditions over the modern time
scale, their known error, their immediate environment, and their
exposure have changed over the years, which must be considered when
studying the climate of centuries past.
Paleoclimatology is the study of past climate over a great period
of the Earth
's history. It uses evidence from
ice sheets, tree rings, sediments, coral, and rocks to determine
the past state of the climate. It demonstrates periods of stability
and periods of change and can indicate whether changes follow
patterns such as regular cycles.
Climate change is the variation in global or regional climates over
time. It reflects changes in the variability or average state of
the atmosphere over time scales ranging from decades to millions of
years. These changes can be caused by processes internal to the
Earth, external forces (e.g. variations in sunlight intensity) or,
more recently, human activities.
In recent usage, especially in the context of environmental policy
, the term "climate
change" often refers only to changes in modern climate, including
the rise in average surface temperature
known as global warming
. In some
cases, the term is also used with a presumption of human causation,
as in the United Nations Framework Convention on Climate Change
UNFCCC uses "climate variability" for non-human caused
Earth has undergone periodic climate shifts in the past, including
four major ice ages
. These consisting of
glacial periods where conditions are colder than normal, separated
accumulation of snow and ice during a glacial period increases the
, reflecting more of the Sun's
energy into space and maintaining a lower atmospheric temperature.
Increases in greenhouse gases
as by volcanic activity, can increase the global temperature and
produce an interglacial. Suggested causes of ice age periods
include the positions of the continents
variations in the Earth's orbit, changes in the solar output, and
Climate models use quantitative methods to simulate the
interactions of the atmosphere
, land surface and ice. They are used
for a variety of purposes from study of the dynamics of the weather
and climate system to projections of future climate. All climate
models balance, or very nearly balance, incoming energy as short
wave (including visible) electromagnetic radiation to the earth
with outgoing energy as long wave (infrared) electromagnetic
radiation from the earth. Any imbalance results in a change in the
average temperature of the earth.
The most talked-about models of recent years have been those used
to infer the consequences of increasing greenhouse gases in the
atmosphere, primarily carbon dioxide
(see greenhouse gas
). These models
predict an upward trend in the global mean surface temperature
with the most rapid increase in temperature being projected for the
higher latitudes of the Northern Hemisphere.
Models can range from relatively simple to quite complex:
- A simple radiant heat transfer model that treats the earth as a
single point and averages outgoing energy
- this can be expanded vertically (radiative-convective models),
- finally, (coupled) atmosphere–ocean–sea
ice global climate models
discretise and solve the full equations for mass and energy
transfer and radiant exchange.
W. Thornthwaite, "An Approach Toward a Rational Classification
of Climate", Geographical Review, 38:55-94, 1948
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